1. Field of the Invention
This invention relates to pneumatic radial tires for heavy vehicles and more particularly to a pneumatic radial tire for heavy vehicles which can efficiently improve its running performance on good roads.
2. Description of the Prior Art
The above mentioned kind of pneumatic radial tire for heavy vehicles such as trucks, buses or the like, which will hereinafter be abbreviated as a truck bus tire, comprises a so-called radial carcass including cords arranged in a radial plane or inclined at a small angle to the radial plane and a belt layer superimposed about the radial carcass and including metal wire cords, particularly steel wire cords. The carcass and belt layer function to reinforce that portion of the tire which is located immediately beneath a tread surface in an extremely rigid manner. As a result, the truck bus tire is far superior in wear resistant property to a conventional bias tire. In addition, the truck bus tire has a number of advantages inclusive of a puncture resistant property and the like. But, the rigid and strong reinforcing effect due to the belt layer results in an uncomfortable ride and makes it difficult to cause the tire to exhibit a tame effect of subordinating it to the road surface. As a result, apart from the use under an off-road condition, the truck bus tire is particularly suitable for use exclusively in running on a good road at a high speed. The truck bus tire for such use has been developed and investigated, while significant improvement in roads such as improved highways has stimulated a great demand for the truck bus tire, thereby eliminating the bias tire from the market.
It is a matter of course that the truck bus tire has been developed and investigated with respect to its internal construction. But, the main development and investigation thereof have been concentrated into improvement in a tread pattern which can cause it to be subordinated to the condition in use of the truck bus tire on a good road at a high speed.
For example, it has heretofore been proposed to provide a rib type tread pattern including a combination of two wide zigzag circumferential grooves located on opposite sides near the equatorial plane of the tire and narrow zigzag grooves substantially parallel to the wide grooves and adapted to be closed at a ground contact region of the tire. Such rib type tread can alleviate so-called uneven wear to be produced in the conventional rib type tread pattern in which the tread is divided into narrower ribs of substantially equal width by means of four wide grooves. But, such rib type tread pattern has the disadvantage that drainage ability of the groove becomes reduced by half and hence the wet resistant property of the tire which is indispensable to the high speed running tire becomes degraded.
The wet resistant property shall be understood to not only mean resistance to the extremely dangerous hydroplane phenomenon which causes the tire to ride on a water film spread on the road surface by a rainfall, for example, and which causes a directional property of the tire to be entirely lost, but also mean resistance to slip and skid which can always renew a biting action of the tire exerted against the water film spread on the road surface both during running and braking of the tire, or the like. Such wet resistant property of the tire can efficiently be improved by shearing the water film spread on the road surface by a step-in side of a tread center region where the tire begins to make contact with the road and by quickly drain water.
As a result, it is rather beneficial to use a lug or block pattern. But, in order to keep a smooth high speed running property, it is more effective to use a block pattern than a lug pattern and more particularly to use a center block-side rib type tread pattern which can favorably maintain the rib's property. In the center block-side rib type tread pattern, that edge of the block which makes contact with the road at first serves to cut the water film and easily drain water through branch grooves for defining the blocks to main groove for separating the blocks from the rib. In addition, the edge of the branch grooves serves to exhibit an efficient road holding ability.
The center block-side rib type tread pattern can easily contract the tread center region in both radial and circumferential directions. As a result, the uneven wear at the shoulder region, that is, shoulder wear produced due to the fact that the crown curvature causes the linear speed on the circumference in the ground contact region of the tire to be smaller toward the tread shoulder can favorably be alleviated by absorbing the difference in the circumferential lengths which induces the uneven wear by the above mentioned contraction in the ground contact region of the tire and by obtaining the smooth ground contact configuration. In addition, it is possible to efficiently alleviate the above mentioned disadvantage inherent to the radial tire that the tire is prevented from becoming subordinated to the road surface and that the tire is uncomfortable in ride.
But, it has been reported that the center block-side rib type tread pattern involves abnormal wear which is different from the above mentioned uneven wear when the tire is used under severe conditions.
Such abnormal wear is inclusive in the first place of a river wear, i.e. a river bank stepped bill-shaped wear which advances from a projection at the edge of the circumferential main groove along the edge, secondly, an irregular wear in which several blocks only become worn in unbalanced state with respect to the other blocks and thirdly a wavy wear selectively produced at the kicking-out side of opposed edges of the side auxiliary groove in the side rib and the like.
Such abnormal wear is also inclusive of heel and toe wear.